Elucidation of potential nanoparticle exposure-disease relationships requires the development of an adequate exposure assessment method, as exposure assessment provides key input into subsequent epidemiologic and toxicological investigation. The absence of a personal exposure assessment method for nanoparticle aerosols prevents progress towards understanding potential nanoparticle exposure-disease relationships. The goal of this work is to develop an accurate, sensitive, and specific method to assess personal exposures to engineered nanoparticles. The method will assess the inhalation route of exposure, and hence, the measurement of nanoparticle concentrations in air. To quantify concentrations of airborne nanoparticles, a measurement must be able to segregate the aerosol by size (to capture only particles smaller than 100 nm) and count these particles following size-segregation. Also, because inhaled nanoparticles enter strictly through the oral/nasal route, the method should be oriented to measure breathing zone concentrations within the vicinity of the upper torso. Finally, and perhaps most importantly, the method must be able to distinguish engineered nanoparticles from other incidental nanoparticles (i.e., ultrafine particles) that are ubiquitous in workplace and ambient air. Relevance Successful completion of this research will establish a method to estimate human exposures to engineered nanoparticles in workplace environments. Such a method is critical to the establishment of nanoparticle dose- response relationships, as current methods lack both specificity and sensitivity. Results from this research can also be translated to the larger realm of health-related air pollution outside of the workplace (e.g., indoor air, ambient air pollution), both nationally and internationally.